This invention relates to a digital/analog conversion apparatus, that is, D/A conversion apparatus, for converting an inputted digital signal into an analog signal and a reproduction apparatus.
Recently, a new optical disk called DVD (Digital Versatile Disc: trademark) which has a capacity greater than that of a compact disk (hereinafter referred to merely as CD: trademark) has been proposed.
The DVD is different from a conventional CD in that information is recorded in a track pitch of 0.8 .mu.m, which is equal to one half the track pitch of 1.6 .mu.m of the conventional CD, on an optical disk of a diameter of 12 cm, and the wavelength of a semiconductor laser is changed from 780 nm for the CD to 630 nm. Further, the EFM (Eight to Fourteen Modulation) modulation method adopted for the CD is realized. Consequently, high density recording corresponding to approximately 4 GB on one face thereof is realized by the DVD.
As one of such DVDs, a multilayer disk having two recording layers has been proposed recently.
As a multilayer disk, a high quality digital audio disk has been proposed by the Applicant of the present application. In the high quality digital audio disk, a 16-bit digital audio signal sampled at 44.1 KHz is recorded in one of the two layers while a 1-bit digital audio signal .SIGMA..DELTA. modulated at 2.8224 MHz, which is a very high sampling frequency equal to 64 times the aforementioned frequency of 44.1 KHz, is recorded in the other layer.
It has been proposed to record a multilayer disk in such a form that data programs of the same music or the like are recorded in the two layers. In particular, a data program of the same contents is recorded as data of ordinary quality of the CD level in one of the two layers and recorded as data of higher quality in the other layer.
In the multilayer disk, since it includes, as one of two layers, a layer in which a 16-bit digital audio signal sampled with 44.1 KHz is recorded, it can be played back also by a compact disk player which is widely available in the market.
Further, a reproduction apparatus which can reproduce data of the new format recorded in the other layer is implemented if a reproduction apparatus such as a CD player is provided with a decoder which can process a 1-bit digital audio signal .SIGMA..DELTA. modulated with the sampling frequency of 2.8224 MHz.
A CD player which includes a decoder which can process a 1-bit digital audio signal allows reproduction from both layers and thus allows reproduction both from a large number of compact disks possessed by users and from the above-described multilayer disk.
A compact disk marketed conventionally and the newly proposed multilayer disk are almost same in appearance. In one of two layers of the multilayer disk, data of a format same as that of a CD, that is, a digital audio signal sampled with 44.1 KHz, quantized with 16 bits and modulated by EFM modulation, is recorded in order to maintain the down compatibility with a compact disk.
It is to be noted that a layer in which data of the CD system is recorded in this manner is hereinafter referred to as CD layer, and the other layer, that is, the layer in which a 1-bit digital audio signal .SIGMA..DELTA. modulated with the sampling frequency of 2.8224 MHz (=64.times.44.1 KHz) is hereinafter referred to as HD (Hi-Definition) layer. Further, data recorded in the CD layer or data recorded on a conventional CD are hereinafter referred to as CD data, and data recorded in the HD layer are hereinafter referred to as HD data.
A generally possible configuration for a reproduction apparatus capable of playing back a CD and a multilayer disk described above is schematically shown in FIG. 1.
A disk 1 shown in FIG. 1 is a CD or a multilayer disk which has a CD layer and an HD layer.
Reading out of data from the disk 1 is performed by an optical head 301 while the disk 1 is driven to rotate by a spindle motor not shown. The data read out by the optical head 301 is converted into a reproduction RF (Radio Frequency) signal by an RF amplifier 302 and supplied to a signal processing circuit 303.
The signal processing circuit 303 decodes the inputted data into 16-bit data sampled with a sampling frequency Fs=44.1 KHz if the input data is CD data, but into 1-bit data, that is, a PDM (Pulse Density Modulation) signal, sampled with 2.8224 MHz (=64.times.44.1 KHz) if the inputted data is HD data. An output of the signal processing circuit 303 is supplied to a D/A conversion section 304.
Description is given of a case wherein the data inputted from the signal processing circuit 303 to the D/A conversion section 304 is CD data. The CD data is inputted to a digital filter 201 of the D/A conversion section 304.
A circuit system from the digital filter 201 to an analog circuit section 204 forms a D/A converter for CD data. The circuit adopts a D/A converter of the PWM (Pulse Width Modulation) system.
The digital filter 201 oversamples the CD data inputted thereto with a frequency according to a magnification set suitably and outputs resulting data to a noise shaper 202.
The noise shaper 202 feeds back noise, that is, a quantization error, upon re-quantization of the input data to, for example, 4 bits to shift the noise spectrum to a higher frequency side outside the audio band to obtain a noise spectrum distribution wherein low frequency side noise is suppressed. Consequently, 4-bit data of the noise spectrum distribution is outputted from the noise shaper 202.
The 4-bit data outputted from the noise shaper 202 are supplied to a PWM (Pulse Width Modulation) conversion section 203 and converted into a signal of a 1-bit waveform as a PWM signal. The resulting signal is inputted to the analog circuit section 204.
The analog circuit section 204 principally performs such signal processing that the inputted signal is passed through a low-pass filter to remove a sampling frequency component of the signal to obtain a continuous analog signal. The analog signal is supplied to a terminal T1 of a switch 206.
Description is given of a case wherein the data inputted from the signal processing circuit 303 to the D/A conversion section 304 is HD data.
Since HD data is a PDM signal whose sampling frequency is 2.8224 MHz equal to 64 times that of a CD and is very high, an analog circuit section 205 is connected directly in the stage next to the signal processing circuit 303. The analog circuit section 205 performs processing suitable for an HD signal, and an analog signal of the HD data is outputted from the analog circuit section 205. The analog signal is outputted to a terminal T2 of the switch 206.
The switch 206 performs switching such that, when a source reproduced is CD data, the terminal T1 and a terminal T3 are connected to each other, but when the source is HD data, the terminal T2 and the terminal T3 are connected to each other. An analog signal of a CD or an analog signal of an HD produced in such a manner as described above is outputted as an analog audio signal to an audio output terminal 207.
In the reproduction apparatus described above with reference to FIG. 1, two circuit systems are provided for CD data and HD data in order to execute conversion into an analog signal and analog signal processing. This principally arises from the fact that different characteristics are required for a low-pass filter and so forth of the analog circuit section between CD data and HD data.
The circuit configuration which includes two circuit systems for CD data and HD data gives rise to a problem that an increased number of parts are required and an increased mounting area is required for circuit components. Further, mutual interference between the two circuits through circuit board patterns for a power supply and the ground occurs, and this gives rise also to a problem that it deteriorates the quality of a reproduction analog audio signal or an analog signal characteristic.
It is known, as a characteristic unique to a PDM signal corresponding to HD data, that some switching distortion is superposed on the PDM signal, for example, by a data output buffer in the process of production of the PDM signal. A distortion which depends upon a signal causes deterioration of a characteristic in a stage after conversion into an analog signal or deterioration of the reproduction sound quality. Therefore, if it is intended, for example, to augment the quality of a reproduction signal, then it is required to remove such a switching distortion as described above and obtain a reproduction signal having fidelity to an original signal.